Exhaust sensor mounting structure

ABSTRACT

The present invention provides an exhaust sensor mounting structure for mounting an exhaust sensor having a sensing element covered by an element cover formed with an exhaust inlet hole to an exhaust pipe of an internal combustion engine. The exhaust pipe includes a tapered section connected to a downstream end of an exhaust purifying device at an upstream end thereof, and a downstream section connected to a downstream end of the tapered section at an upstream end thereof. The exhaust sensor mounting structure includes a guide member, and a fixing member fixing the guide member in such a position within the exhaust pipe that water vapor contained in an exhaust gas of the internal combustion engine and condensed upstream of the guide member is guided to fly through an area in which the exhaust inlet hole of the element cover located inside the exhaust pipe is not located.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to Japanese Patent Application No. 2006-49824 filed on Feb. 27, 2006, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust sensor mounting structure for mounting an exhaust sensor to an exhaust pipe of an internal combustion engine, especially to an exhaust sensor mounting structure suitable for use in an exhaust purifying system of a diesel engine.

2. Description of Related Art

In recent years, regulations of exhaust emissions from diesel engine-mounted vehicles are becoming stringent in view of earth environment protection. To address such regulations, it is known to detect oxygen concentration in an exhaust gas, and feedback the detected oxygen concentration to an engine control unit to eliminate variation of a fuel injection amount and an EGR (Exhaust Gas Recirculation) amount, so that nitrogen oxide, and particulate carbon or smoke in the exhaust gas can be reduced. It is common that, as an exhaust sensor for detecting oxygen concentration in an exhaust gas, a sensor of the zirconia solid electrolyte type that utilizes oxygen ion pumping action of the zirconia solid electrolyte. Such a sensor of the zirconia solid electrolyte type has to be heated over 650° C. to produce an accurate sensor output, because it uses a zirconia solid electrolyte in its sensing section. Accordingly, it includes therein an electric heater.

However, the sensor as described above poses a problem when it is mounted to an exhaust pipe of an engine as described below. When the engine is started from cold state, since an inner wall temperature of the exhaust pipe is low, water vapor contained in an exhaust gas is condensed on the inner wall of the exhaust pipe. This condensed water flies off the inner wall due to the flow of the exhaust gas, and spatters on the sensing section of the sensor which is being heated by the electric heater included therein. This applies a large thermal stress to the sensing section of the sensor, as a result of which the zirconia solid electrolyte forming the sensing section of the sensor can be broken. If the zirconia solid electrolyte is broken, not only the accuracy of the sensor output becomes worse, but also an exhaust purifying system using the sensor output of the sensor malfunctions. To prevent this, it is necessary to start supplying electric power to the electric heater after the water vapor that has been condensed on the inner wall of the exhaust pipe since the start of the engine is dried off. Accordingly, the sensor cannot be put into operation until a fair amount of time has elapsed since the start of the engine.

Incidentally, there are known various sensor mounting structures designed to suppress the condensed water vapor from spattering on the sensing section. For example, Japanese Patent Application Laid-open No. 2004-124783 discloses a sensor mounting structure in which a specific part of an exhaust pipe of an engine is located in a position lower than other parts, an exhaust sensor is mounted in a position higher than a floor portion of this specific part, and a reservoir is provided in a position lower than the floor portion, the reservoir and the exhaust pipe being communicated to each other at a position upstream of a sensor mounting position and a position downstream of the sensor mounting position. This structure makes it possible to collect water vapor condensed in the exhaust pipe after the engine is stopped in the reservoir, so that when the engine is restarted, the condensed water can be prevented from spattering onto the sensing section of the sensor. However, since it takes time for the whole of the condensed water to be collected in the reservoir, if the engine is restarted in a state where some condensed water remains in the inside of the exhaust pipe, there is a possibility the remaining condensed water spatters onto the sensing section of the sensor.

For another example, Japanese Patent Application Laid-open No. 2005-127214 discloses a sensor mounting structure in which an exhaust pipe is provided with an extended section having a larger diameter, and an exhaust sensor is mounted in a position downstream of an upstream side end of the extended section and higher than a floor portion of the extended section. This structure is indented to reduce a flow velocity of an exhaust gas at the extended section, to thereby shorten a flying distance of the condensed water in the exhaust pipe. However, it has been found that the flow velocity of the exhaust gas is lowered only near the inner wall of the extended section, and is hardly lowered near an axial center of the extended section. Accordingly, there is a possibility that the condensed water is sucked by the exhaust gas flowing through the axial center of the exhaust pipe, and spatters onto the sensing section of the sensor.

As explained above, the conventional sensor mounting structures cannot reliably prevent the water vapor condensed in the exhaust pipe from spattering onto the sensing section of the exhaust sensor, and accordingly it has bee difficult to normally control the engine when the engine is started in a state where some condensed water remains on the inner wall of the exhaust pipe.

SUMMARY OF THE INVENTION

The present invention provides an exhaust sensor mounting structure for mounting, to an exhaust pipe of an internal combustion engine provided with an exhaust purifying device in a midway portion of the exhaust pipe, an exhaust sensor having a sensing element covered by an element cover formed with an exhaust inlet hole, the exhaust pipe including a tapered section connected to a downstream end of the exhaust purifying device at an upstream end thereof, and a downstream section connected to a downstream end of the tapered section at an upstream end thereof, the tapered section having a diameter narrowing towards the downstream end thereof, the exhaust sensor being mounted to the downstream section, the exhaust sensor mounting structure comprising:

a guide member; and

a fixing member fixing the guide member in such a position within the exhaust pipe that water vapor contained in an exhaust gas of the internal combustion engine and condensed upstream of the guide member is guided to fly through an area in which the exhaust inlet hole of the element cover located inside the exhaust pipe is not located.

In the present invention, the flying path, of the condensed water is totally changed by the guide member in order that the condensed water is prevented from entering through the exhaust inlet hole of the element cover. Accordingly, with the present invention, the condensed water can be reliably prevented from spattering onto the sensing element of the exhaust sensor. The present invention makes it possible to normally start a vehicle engine immediately after water vapor contained in the exhaust gas of the engine is condensed in the exhaust pipe in a case where the exhaust sensor is an oxygen concentration sensor including a zirconia solid electrolyte in its sensing element which should be heated above 650° C. during use.

The guide member may have a hollow cylindrical shape and be disposed in the downstream section coaxially with downstream section, such that an upstream end of the guide member is located upstream of an intersecting position at which a first plane extending from the downstream end of the tapered section at an angle of 30 degrees with an inner wall surface of the downstream section and an inner wall surface of the guide member intersect with each other, and a downstream end of the guide member is located in such a position that the exhaust inlet hole of the element cover is located upstream of a second plane extending from the downstream end of the guide member at an angle of 30 degrees with an outer wall surface of the guide member.

The upstream end of the guide member may be located upstream of the downstream end of the tapered section.

The downstream end of the guide member may be located downstream of a mounting position of the exhaust sensor.

The exhaust sensor mounting structure may further comprise a tapered guide member having a circular cross section and connected to the upstream end of the guide member at a downstream end thereof, and extending into the tapered section, the tapered guide member having a diameter expanding toward an upstream end thereof.

An outer edge of the upstream end of the guide member or the tapered guide member may be chamfered.

The exhaust sensor may be an oxygen concentration sensor including a zirconia solid electrolyte type sensing element, and an electric heater for heating the zirconia solid electrolyte type sensing element.

The exhaust purifying device may be a diesel particulate filter.

Other advantages and features of the invention will become apparent from the following description including the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram schematically showing an overall structure of a diesel engine which can be applied with an exhaust sensor mounting structure of the invention;

FIG. 2 is a longitudinal cross-sectional view of an exhaust sensor of the zirconia solid electrolyte type;

FIG. 3 is a partially enlarged cross-sectional view of the exhaust sensor cut along a line A-A shown in FIG. 2.

FIG. 4 is a front cross-sectional view of an exhaust sensor mounting structure according to a first embodiment of the invention;

FIG. 5 is a cross section of the exhaust sensor mounting structure as viewed in the direction of arrows A in FIG. 4;

FIG. 6 is an enlarged view of a circled area B shown in FIG. 4;

FIG. 7 is an enlarged view of a circled area C shown in FIG. 4;

FIG. 8 is a front cross-sectional view of an exhaust sensor mounting structure according to a second embodiment of the invention;

FIG. 9 is a front cross-sectional view of an exhaust sensor mounting structure according to a third embodiment of the invention; and

FIGS. 10A to 10C are partially enlarged cross-sectional views of an exhaust sensor mounting structure according to a fourth embodiment of the invention around an upstream end of a guide member;

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 is a diagram schematically showing an overall structure of a diesel engine 1 which can be applied with an exhaust sensor mounting structure of the invention. The diesel engine 1 is a four-cylinder engine having a common rail 2, and four injectors 3 coupled to the common rail 2 and operating to respectively inject fuel into a combustion chamber of a corresponding one of four cylinders of the diesel engine 1. The diesel engine 1 has also an intake manifold 4 coupled to an intake pipe 5. A flow rate of an intake air is adjusted by an intake throttle valve 6. The intake air that passes through the intake pipe 5 is filtered by an air cleaner 7, and detected by an air flow meter 8.

The exhaust gas from the engine 1 is discharged through an exhaust passage 9. The exhaust passage 9 includes an exhaust manifold 10, and an exhaust pipe 11 provided with a diesel particulate filter (referred to as a DPF hereinafter) 12 at a midway portion thereof. The DPF 12 may be a conventional one that can be manufactured by forming a heat-resisting porous ceramic material such as silicon carbide or cordierite to have a plurality of first cells and second cells disposed in parallel to the first cells, the first cells being opened at their upstream ends and closed at their downstream ends, while the second cells being closed at their upstream ends and opened at their downstream ends. The exhaust gas enters the DPF 12 from the opened ends of the first cells, and particulates contained in the exhaust gas are captured in porous walls of these cells when the exhaust gas moves to the second cells. The surfaces of the porous walls may carry a catalyst that promotes oxidation of the captured particulate, so that the captured particulate can be burnt off periodically.

The exhaust pipe 11 is provided with a turbine 14 of a turbo charger 13 at upstream of the DPF 12. The turbocharger is coupled to a compressor 15 provided in the intake pipe 5 through a turbine shaft thereof, in order that the compressor 15 is driven by kinetic energy of the exhaust gas, to thereby compress the intake air sucked into the intake pipe 5. The intake pipe 5 is provided with an intercooler 17 at upstream of the intake throttle valve 6 in order to cool the intake air whose temperature has risen by being compressed by the compressor 15. The exhaust pipe 11 is also provided with, as an exhaust sensor, an oxygen concentration sensor 18 of the zirconia solid electrolyte type at downstream of the DPF 12 in order to detect oxygen concentration in the exhaust gas. The structure and operation of the exhaust sensor 18 are described later.

The exhaust manifold 10 is coupled to the intake manifold 4 through an EGR passage 19, so that part of the exhaust gas is returned to the air intake side. The EGR passage 19 is provided with an EGR valve 20 at its outlet end which opens to the intake manifold 4. An amount of the exhaust gas returning to the air intake side (referred to as “EGR gas” hereinafter) can be adjusted by controlling the opening degree of the EGR valve 20. The EGR passage 19 is provided with an EGR cooler 21 at its midway portion to cool the EGR gas. The reference numeral 22 denotes an intake air pressure sensor for detecting a pressure of the intake air in the intake manifold 4.

The reference numeral 23 denotes an ECU (Electronic Control Unit) that receives output signals from the airflow meter 8, intake air pressure sensor 22, and exhaust sensor 18. The ECU 23 further receives output signals from other not shown sensors, such as an engine speed sensor, a vehicle speed sensor, a cooling water sensor, an accelerator opening degree sensor, a crank position sensor, a fuel pressure sensor. The ECU 23 determines the running state of the engine 1 based on these output signals, and calculates an optimum fuel injection amount and an optimum amount of the EGR gas depending on the determined running state of the engine 1, in order to feedback-controls the intake throttle valve 6, injectors 3, EGR valve 20, turbocharger 13, etc.

Next, the structure and operation of the exhaust sensor 18 are explained with reference to FIG. 2 and FIG. 3. FIG. 2 is a longitudinal cross-sectional view of the exhaust sensor 18, and FIG. 3 is a partially enlarged cross-sectional view of the exhaust sensor 18 cut along a line A-A shown in FIG. 2. The exhaust sensor 18 is an oxygen concentration sensor of the zirconia solid electrolyte type. The exhaust sensor 18 has a tubular housing 40 made of metal, and housing therein a sensing element 43 including a zirconia solid electrolyte sheet 42, and a tubular insulating member 41 holding the zirconia solid electrolyte sheet 42 at a midway peripheral portion of the zirconia solid electrolyte sheet 42. The insulating member 41 is formed with a hollow at one end portion thereof. This hollow is filled with an insulating seal member 44 to isolate the exhaust gas in the exhaust pipe 11 from the atmospheric air.

A front end portion (downward end portion in FIG. 2) of the sensing element 43, which serves as an oxygen concentration detecting portion 45, protrudes from the housing 40, and is housed in a case-like inner element cover 46 which is contained in a case-like outer element cover 47. The inner element cover 46 has exhaust inlet holes 46 a formed at its peripheral surface to take in the exhaust gas therein. Likewise, the outer element cover 47 has exhaust inlet holes 47 a formed at its peripheral surface to take in the exhaust gas therein. The exhaust inlet holes 46 a and the exhaust inlet holes 47 a are located such that that they do not face to each other. More specifically, the exhaust inlet holes 47 a are located at positions closer to the front end of the exhaust sensor 18 than the positions at which the exhaust inlet holes 46 a are located. That is, the exhaust inlet holes 47 a are located downward of the exhaust inlet holes 46 a as shown in FIG. 9. In this embodiment, the exhaust inlet holes 47 a are located at a distance of about 7/10 of the radius of the exhaust pipe 11 from the inner wall surface of the exhaust pipe 11.

A rear end portion (upward end portion in FIG. 2) of the sensing element 43 protrudes from the housing 40, and is housed in a tubular atmosphere cover 48 fixed to a rear end of the housing 40. The atmosphere cover 48 has an outer tube 48 c, and an inner tube 48 d, so that it has a double walled structure at its upper half portion. The outer tube 48 c is formed with atmospheric air inlet holes 48 a. The inner tube 48 d is formed with atmospheric air inlet holes 48 b each of which faces a corresponding one of the atmospheric air inlet holes 48 a. The atmospheric air is taken in inside the atmosphere cover 48 through the atmospheric air inlet holes 48 a, 48 b, and led to a reference electrode 42 b contained in the sensing element 43. A water-shedding filter is 49 is disposed between the outer tube 48 c and the inner tube 48 d at a position where the atmospheric air inlet holes 48 a, 48 b are formed, so that water can be prevented from entering the exhaust sensor 18.

A ring-like insulating support member 50 is disposed inside the atmosphere cover 48. The ring-like insulating support member 50 includes therein plate-spring-like metal terminals 52, 53 one ends of which are located at a lead portion (rear end portion) 51 of the sensing element 43, and the other end of which are respectively electrically connected to an external lead 54, and an external lead 55. The insulating support member 50 is formed with a communication hole 50 a at its upper end portion so that the atmospheric air taken in through the atmospheric air inlet holes 48 a, 48 b can reach an atmosphere passage 57 formed by an atmospheric air inlet duct 56 included in the sensing element 43.

It should be noted that the exhaust sensor 18 has two external leads 54, two external leads 55, two metal terminals 52, and two metal terminals 53, although only one of them is shown for each of them in FIG. 9. That is, the exhaust sensor 18 has four external leads, and four metal terminals. The metal terminals 53 connected to a corresponding one of the external leads 55 shown in the left side of FIG. 9 are respectively connected to a working electrode 42 a (to be described later) and to the reference electrode 42 b. On the other hand, the metal terminals 52 connected to a corresponding one of the external leads 54 shown in the right side of FIG. 2 are respectively connected to a pair of leads of an electric heater 58 (to be described later).

As shown in FIG. 3, the sensing element 43 includes the zirconia solid electrolyte sheet 42, the working electrode 42 a disposed on an outer side surface of the zirconia solid electrolyte sheet 42 so as to be exposed to the exhaust gas, the reference electrode 42 b disposed on an inner side surface of the zirconia solid electrolyte sheet 42. As explained above, the zirconia solid electrolyte sheet 42 is electrically connected to the metal terminals 53 at its lead end portion 51. The reference electrode 42 b faces the atmosphere passage 57 formed by the atmospheric air inlet duct 56.

The working electrode 42 a and the reference electrode 42 b facing each other across from the zirconia solid electrolyte sheet 42 constitute an electrochemical cell. The internal resistance of the electrochemical cell has to be sufficiently low for the electrochemical cell to produce an accurate output signal. Accordingly, the oxygen concentration detecting portion 45 of the sensing element 43 has to be heated over 650° C. Accordingly, the exhaust sensor 18 is provided with the electric heater 58 folded and embedded in an insulating sheet 59. The electric heater 58 is supplied with electric power through a pair of the external lead 54 and the metal terminal 52.

Other than the working electrode 42 a, an exhaust-transmitting layer 60 and an exhaust-shielding layer 61 are laminated on the outer side surface of the zirconia solid electrolyte sheet 42. The exhaust-transmitting layer 60, which serves to introduce the exhaust gas to the working electrode 42 a, is a porous sheet that can be made by sheet-forming of ceramics such as alumina, spinel, or zirconia. The sensing element 43 is covered by a protection layer 62 made of alumina having a high specific surface area in its entirety, so that the exhaust-transmitting layer 60 can be prevented from being clogged by poisoning components contained in the exhaust gas.

The exhaust sensor 18 having the above described structure is mounted to the exhaust pipe 11 by screwing external screw threads 40 a formed in a lower end portion of the housing 40 into internal screw threads formed in a screw base 11 c provided in the exhaust pipe 11. A gasket 63 is inserted between the housing 40 and the screw base 11 c in order to prevent the exhaust sensor 18 from loosening from the exhaust pipe 11 and preventing the exhaust gas from leaking due to vibration.

The exhaust gas enters the outer element cover 47 from the exhaust inlet holes 47 a, enters the inner element cover 46 from the exhaust inlet holes 46 a, and passes through the exhaust-transmitting layer 60 to reach the zirconia solid electrolyte sheet 42. When a certain voltage is applied between the reference electrode 42 b exposed to the atmospheric air in the atmosphere passage 57 and the working electrode 42 a exposed to the exhaust gas, a limiting current flow between these electrodes 42 a, 42 b depending on an oxygen concentration of the exhaust gas. This limiting current is outputted to the ECU 23 through the external leads 55 as a signal indicative of the oxygen concentration of the exhaust gas in the exhaust pipe 11.

Next, an exhaust sensor mounting structure according to a first embodiment of the invention is explained with reference to FIGS. 4 to 7. FIG. 4 is a front cross-sectional view of the exhaust sensor mounting structure of this embodiment, FIG. 5 is a cross section of this exhaust sensor mounting structure as viewed in the direction of arrows A in FIG. 4, FIG. 6 is an enlarged view of a circled area B shown in FIG. 4 and FIG. 7 is an enlarged view of a circled area C shown in FIG. 4. As shown in these figures, the exhaust pipe 11 made of metal and having a cylindrical shape is provided with the DPF 12 as an exhaust purifying device. The diameter of the exhaust pipe 11 is large at a portion in which the DPF 12 is mounted compared to other portions, because the DPF 12 has a large effective exhaust passage area, and accordingly has a large outer diameter. Accordingly, a section 11 a of the exhaust pipe 11 immediately downstream of the DPF 12 makes a tapered section 11 b which is connectable to a downstream section 11 h of the exhaust pipe 11 that follows the tapered section 11 b in the downstream direction. The exhaust sensor 18 of the zirconia solid electrolyte type is screw-mounted to the screw base 11 c provided in the exhaust pipe 11. The exhaust inlet holes 47 a of the outer element cover 47 are located at a distance of approximately 7/10 of the radius of the exhaust pipe 11 from the inner wall surface of the exhaust pipe 11.

The high-temperature exhaust gas that has been purified to remove the particulate carbon or smoke therefrom by the DPF 12 flows downstream through the exhaust pipe 11 (in the rightward direction in FIG. 4). When the engine is stopped during the cold season, water vapor contained in the exhaust gas remaining in the exhaust pipe 11 may condense on the inner wall surface of the exhaust pipe 11 that has been cooled by a low-temperature atmospheric air. Also, when the engine is started from cold state, water vapor contained in the exhaust gas may condense on the low-temperature inner wall surface of the exhaust pipe 11. The condensed water can adhere to the inner wall surface of the tapered section 11 b of the exhaust pipe 11 as a matter of course. The condensed water adhered to the inner wall surface of the tapered section 11 b flies along the inner wall surface towards the downstream section 11 h of the exhaust pipe 11 by the action of the flow of the exhaust gas. In an area in which the tapered section 11 b and the downstream section 11 h are connected to each other, the exhaust gas flowing along the inner wall of the tapered section 11 b gradually changes to flow along an axial direction of the exhaust pipe 11 as shown by a curved solid arrow in FIG. 4, by the action of which the condensed water flies along a curved dotted arrow shown in FIG. 4 in this area. Accordingly, if a guide member 30 described below is not provided, the condensed water tends to fly along near an axial center of the exhaust pipe 11 at an area in which the exhaust sensor 10 is mounted, and the oxygen concentration detecting portion 45 is therefore easily spattered by the condensed water that has entered through the exhaust inlet holes 47 a of the outer element cover 47.

The guide member 30 is made of thin metal plate having heat resistance and corrosion resistance, and has a hollow cylindrical shape. This guide member 30 is disposed coaxially with the downstream section 11 h of the exhaust pipe 11, and welded to the exhaust pipe 11 at its three circumferential positions through fixing members 31. As shown in FIG. 5, the distance between an outer surface of the guide member 30 and the inner surface of the exhaust pipe 11 is set at a constant value of t along their circumferences. The fixing member 31 may be made of the same material as the guide material 30. A metal wool may be charged in the circumferential space between the guide member 30 and the exhaust pipe 11. The guide member 30 may be made of metal mesh, or punching metal. The DPF 12 may be replaced by a catalyst converter.

The inventor carried out experiment to find out in which position the guide member 30 should be located to effectively prevent the condensed water from entering through the exhaust inlet holes 47 a of the outer element cover 47. In this experiment, critical positions of an upstream end 30 a of the guide member 30 to prevent the condensed water from entering inside the guide member 30 were plotted for various diameters of the guide member 30. As a result, a plot plane X shown in FIG. 6 was obtained. The plot plane X extends from a downstream end lie of the tapered section 11 b at an angle of 30 degrees with the inner wall surface of the downstream section 11 h of the exhaust pipe 11. From this experiment, it has been found that the upstream end 30 a of the guide member 30 has to be located upstream of an intersecting position M shown in FIG. 6 at which the plot plane X and an inner wall surface of the guide member 30 having a given diameter intersect with each other. From this experiment, it has been also found that the flying path of the condensed water flying from the tapered section 11 b to the downstream section 11 h of the exhaust pipe 11 varies little even when the angle of inclination of the tapered section 11 b is varied, because of the high flow velocity of the exhaust gas.

In addition to the above, critical positions of the downstream end 30 b of the guide member 30 to prevent the condensed water that has passed over the guide member 30 while being varied in its flying path from entering through the exhaust inlet holes 47 a of the outer element cover 47 were plotted for various diameters of the guide member 30. As a result, a plot plane Y shown in FIG. 6 was obtained. The plot plane Y extends from the downstream end 30 b of the guide member 30 at an angle of 30 degrees with an outer wall surface 30 c of the guide member 30. From this experiment, it has been found that the exhaust inlet holes 47 a of the outer element cover 47 have to be located upstream of the plot plane Y.

As explained above, previously, the condensed water adhered to the inner wall surface of the tapered section 11 b flies along the dotted curved arrow entering an exhaust gas passing area W shown in FIG. 4. On the other hand, by the provision of the guide member 30, the condensed water adhered to the inner wall surface of the tapered section 11 b flies along the solid curved arrow entering a condensed water passing area Z shown in FIG. 4 that resides in the space between the exhaust pipe 11 and the guide member 30. The condensed water that has passed through the condensed water passing area Z, and passed over the downstream end 30 b of the guide member 30 is sucked towards the exhaust gas passing area W. However, the condensed water can be prevented from entering through the exhaust inlet holes 47 a if the exhaust inlet holes 47 a are located upstream of the plot plane Y.

In the conventional methods, prevention of the water spattering onto the sensing element of the exhaust sensor is performed by collecting the condensed water in a reservoir, or by reducing the flow velocity of the condensed water. In this embodiment, the flying path of the condensed water is totally changed by the guide member 30 in order that the condensed water is prevented from entering through the exhaust inlet holes 47 a of the outer element cover 47. Accordingly, with this embodiment, the condensed water can be reliably prevented from spattering onto the oxygen concentration detecting portion 45 of the exhaust sensor 18. This makes it possible to reliably prevent the zirconia solid electrolyte included in the oxygen concentration detecting portion 45 which should be heated above 650° C. during use from being broken due to thermal stress. Accordingly, this embodiment makes it possible to normally start a vehicle engine immediately after water vapor contained in the exhaust gas of the engine is condensed in the exhaust pipe. Since the length of the guide member 30 can be made sufficiently small on the condition that it provides the above described effects, this embodiment can be provided at low cost.

Incidentally, the experiment shows that the distance t between the outer surface of the guide member 30 and the inner wall surface of the exhaust pipe 11 has to be at least 0.5 mm for the condensed water to be able to pass through the space between the guide member 30 and the exhaust pipe 11. The distance t can be increased (or the diameter of the guide member 30 can be reduced) to such a value that the guide member 30 can provide the above described effects.

FIG. 8 shows a second embodiment of the invention. The second embodiment is different from the first embodiment in that the upstream end 30 a and the downstream end 30 b of the guide member 30 are extended upwardly and downwardly, respectively, so that the upstream end 30 a is located upstream of the downstream end lie of the tapered section 11 b, and the downstream end 30 b is located downstream of the position at which the exhaust sensor 18 is mounted. In this embodiment, the guide member 30 is formed with a hole 30 c through which the outer element cover 47 in which the oxygen concentration detecting portion 45 of the exhaust sensor 18 is housed is allowed to pass.

The second embodiment makes it possible to prevent the condensed water from spattering onto the oxygen concentration detecting portion 45 of the exhaust sensor 18 more reliably than the first embodiment, since the upstream end 30 a of the guide member 30 is extended upstream beyond the downstream end 11 e of the tapered section 11 b, and the downstream end 30 b of the guide member 30 is extended downstream beyond the mounting position of the exhaust sensor 18, so that the condensed water does not fly through the exhaust gas passing area W in which the outer element cover 47 covering the oxygen concentration detecting portion 45 is located, but is guided downstream beyond the mounting position of the exhaust sensor 18. Since a relatively large part of the condensed water flows along a lower portion of the exhaust pipe 11, a slit extending along the entire length of the guide member 30 through which the oxygen concentration detecting portion 45 covered by the outer element cover 47 is allowed to pass may be formed instead of the hole 30 c. The upstream end 30 a of the guide member 30 may be further extended upstream as shown by a phantom line (chain double-dashed line) shown in FIG. 8 in order to completely shift the flying path of the condensed water away from the exhaust gas passing area W. The extension length of the guide member 30 may be determined depending on the flow velocity of the exhaust gas.

FIG. 9 shows a third embodiment of the invention. In this embodiment, the upstream end 30 a of the guide member 30 is located at the position of the downstream end 11 e of the tapered section 11 b, and a tapered guide member 30 d is connected to the guide member 30 at a downstream end thereof such that it extends from the upstream end 30 a of the guide member 30 into the tapered section 11 b. The tapered guide member 30 d has a circular cross section and a diameter expanding toward its upstream end. The inclination of the tapered guide member 30 d is the same as that of the tapered section 11 b, and accordingly their surfaces are parallel to each other. The material of the tapered guide member 30 d is the same as that of the guide member 30. The tapered guide member 30 d may be made integrally with the guide member 30, or it may be made separately from the guide member 30, and welded to the guide member 30. As for the rest, the third embodiment is the same as the second embodiment in structure.

In the third embodiment, the exhaust gas is guided to flow along the inclined surface of the tapered guide member 30 d so that the condensed water is brought to the condensed water passing area Z. The length of the tapered guide member 30 d may be determined depending on the flow velocity of the exhaust gas.

FIG. 10A to 10C are partially enlarged cross-sectional views of a fourth embodiment of the invention around the upstream end 30 a of the guide member 30. FIG. 10A shows a case where a 45-degree chamfer 30 e is formed at an outer edge of the upstream end 30 a of the guide member 30 by cutting work or pressing work. FIG. 10B shows a case where an R-shaped chamfer 30 f is formed at the outer edge of the upstream end 30 a of the guide member 30 by cutting work or pressing work. FIG. 10C shows a case where an R-shaped chamfer 30 f and an R-shaped protrusion 30 g are formed respectively at the outer edge and an inner edge of the upstream end 30 a and of the guide member 30. In this case, the R-shaped chamfer 30 f and the R-shaped protrusion 30 g can be formed at the same time when the guide member 30 is cut at the upstream end 30 a thereof by a parting roller.

The provision of the chamfer 30 e, or 30 f at the outer edge of the upstream end 30 a of the guide member 30 makes it possible to reduce a flow resistance of the exhaust gas at the upstream end 30 a of the guide member 30, to thereby more reliably shift the flying path of the condensed water to the condensed water passing area Z. The provision of the protrusion 30 g at the inner edge of the upstream end 30 a of the guide member 30 makes it possible to prevent the condensed water from entering the exhaust gas passing area W. The chamfer 30 e and the chamfer 30 f may be formed at an outer edge of an upstream end of the tapered guide member 30 d shown in FIG. 9.

Although the exhaust sensor 18 is described as being an oxygen concentration sensor of the zirconia solid electrolyte type, it may be a nitrogen oxide concentration sensor, or a hydrocarbon concentration sensor, or a carbon monooxide concentration sensor to be mounted downstream of the tapered section 11 b. The above described embodiments are directed to an exhaust purifying system of a diesel engine, however, the present invention is applicable to an exhaust purifying system of a gasoline engine

The above explained preferred embodiments are exemplary of the invention of the present application which is described solely by the claims appended below. It should be understood that modifications of the preferred embodiments may be made as would occur to one of skill in the art. 

1. An exhaust sensor mounting structure for mounting, to an exhaust pipe of an internal combustion engine provided with an exhaust purifying device in a midway portion of said exhaust pipe, an exhaust sensor having a sensing element covered by an element cover formed with an exhaust inlet hole, said exhaust pipe including a tapered section connected to a downstream end of said exhaust purifying device at an upstream end thereof, and a downstream section connected to a downstream end of said tapered section at an upstream end thereof, said tapered section having a diameter narrowing towards said downstream end thereof, said exhaust sensor being mounted to said downstream section, said exhaust sensor mounting structure comprising: a guide member; and a fixing member fixing said guide member in such a position within said exhaust pipe that water vapor contained in an exhaust gas of said internal combustion engine and condensed upstream of said guide member is guided to fly through an area in which said exhaust inlet hole of said element cover located inside said exhaust pipe is not located.
 2. The exhaust sensor mounting structure according to claim 1, wherein said guide member has a hollow cylindrical shape and is disposed in said downstream section coaxially with downstream section, an upstream end of said guide member being located upstream of an intersecting position at which a first plane extending from said downstream end of said tapered section at an angle of 30 degrees with an inner wall surface of said downstream section and an inner wall surface of said guide member intersect with each other, a downstream end of said guide member being located in such a position that said exhaust inlet hole of said element cover is located upstream of a second plane extending from said downstream end of said guide member at an angle of 30 degrees with an outer wall surface of said guide member.
 3. The exhaust sensor mounting structure according to claim 2, wherein said upstream end of said guide member is located upstream of said downstream end of said tapered section.
 4. The exhaust sensor mounting structure according to claim 3, wherein said downstream end of said guide member is located downstream of a mounting position of said exhaust sensor.
 5. The exhaust sensor mounting structure according to claim 2, further comprising a tapered guide member having a circular cross section and connected to said upstream end of said guide member at a downstream end thereof, and extending into said tapered section, said tapered guide member having a diameter expanding toward an upstream end thereof.
 6. The exhaust sensor mounting structure according to claim 2, wherein an outer edge of said upstream end of said guide member is chamfered.
 7. The exhaust sensor mounting structure according to claim 5, wherein an outer edge of said upstream end of said tapered guide member is chamfered.
 8. The exhaust sensor mounting structure according to claim 1, wherein said exhaust sensor is an oxygen concentration sensor including a zirconia solid electrolyte type sensing element, and an electric heater for heating said zirconia solid electrolyte type sensing element.
 9. The exhaust sensor mounting structure according to claim 1 configured to be applied to an exhaust pipe of a diesel engine with an exhaust purifying device.
 10. The exhaust sensor mounting structure according to claim 1, wherein said exhaust purifying device is a diesel particulate filter. 